Lactation has immediate favorable effects on glucose tolerance that may persist after weaning to prevent development of type 2 diabetes mellitus (DM) in women. Development of glucose intolerance during pregnancy, known as gestational diabetes (GDM), is a strong predictor of subsequent development of type 2 DM. In the U.S., GDM occurs in about 3-5% of all pregnancies and affects 135,000 women per year. Within 5 years after delivery, 20 to 50% of women with previous GDM will develop type 2 DM. Risk factors for type 2 DM after GDM pregnancy include pancreatic b-cell dysfunction, severity of glucose intolerance during pregnancy and early postpartum periods, family history of DM, maternal obesity, recurrent GDM and weight gain. Lactation lowers maternal glucose and insulin levels, improves glucose tolerance and the disposition index, indicating better-cell function for their degree of insulin resistance. Whether the intensity and duration of lactation reduces subsequent risk of DM has rarely been examined. Previous studies are limited by qualitative measures of lactation (ever vs. never), current status (yes vs. no), or lifetime months of lactation. No prospective studies have assessed lactation intensity (supplemental feedings) and/or duration in relation to development of type 2 DM after recent GDM pregnancy. We propose a prospective cohort study that will enroll 1,098 women with a recent GDM diagnosis who are free of type 2 DM at 6-8 weeks postpartum in the Kaiser Permanente Health Care System in northern California. Women will be followed annually for two years to administer the 2 hr 75 g oral glucose tolerance test. We will determine whether prolonged, intensive lactation compared with no lactation is associated with a lower incidence of type 2 DM, and more favorable changes in fasting and 2-hr post load plasma glucose and insulin levels, indices of insulin resistance, body weight, waist girth and plasma adiponectin. Because randomization is not feasible, the rigorous assessment of lactation intensity and duration controlling for multiple potential confounders is the most valid study design. The potential public health impact of these findings for women with a history of GDM is substantial given that less than 50% of U.S. women lactate for four months. Lactation is a modifiable behavior that may be translated into a practical, low-cost intervention that has the potential to enhance postpartum interventions that promote healthy dietary intakes and increased physical activity.